Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch and the position of the touch on the touch sensor panel, and the computing system can then interpret the touch in accordance with the display appearing at the time of the touch, and thereafter can perform one or more actions based on the touch. In the case of some touch sensing systems, a physical touch on the display is not needed to detect a touch. For example, in some capacitive-type touch sensing systems, fringing electrical fields used to detect touch can extend beyond the surface of the display, and objects approaching near the surface can be detected near the surface without actually touching the surface.
Capacitive touch sensor panels can be formed from a matrix of drive and sense lines of a substantially transparent conductive material, such as Indium Tin Oxide (ITO), often arranged in rows and columns in horizontal and vertical directions on a substantially transparent substrate. It is due in part to their substantial transparency that capacitive touch sensor panels can be overlaid on a display to form a touch screen, as described above. Some touch screens can be formed by partially integrating touch sensing circuitry into a display pixel stack-up (i.e., the stacked material layers forming the display pixels).
Integrating touch sensing circuitry with display circuitry can require high frequency switching of voltage levels to accommodate the different voltage requirements of touch sensing and display modes. Operational transconductance amplifiers (OTAs) can be used with high-capacitance touch sensing and display circuitry because OTAs can maintain a high DC gain and accuracy while maintaining a low noise characteristic and avoiding the compensation complexity and area penalty of multiple stage amplifiers.
One figure of merit used to measure amplifier performance is slew rate, or the maximum rate of change of output voltage with respect to time. High slew rate can be especially important in applications which require high frequency switching of voltage levels, such as in integrated touch sensing and display circuitry. Slew rate can be increased by increasing the OTA bias current, but this method increases the amplifier's quiescent supply current and therefore power consumption.